Gas turbines must be driven to rotate at a starting speed by auxiliary means prior to fuel injection and ignition and self-sustained operation. In the past, for example, gear box systems driven by auxiliary electric or compressed air motors have been used to rotate the turbine to starting speed. xe2x80x9cAirxe2x80x9d impingement starting systems have also been used with small turbines and operated by directing a stream of gas, typically air, onto the turbine or compressor wheel to cause rotation of the main rotor. These prior art systems are complex and difficult to implement.
Electrical power may be generated by using a gas turbine to drive an alternator. The alternator may be driven by a free turbine which is coupled to the rotor of the alternator or through a gear box. In these systems, the speed of the turbine must be precisely controlled to maintain the desired frequency and voltage of the alternating current output.
In accordance with the present invention, an alternator having a permanent magnet rotor is connected to the main turbine rotor making possible both starting of the turbine as well as generation of electrical power. The electrical system described herein allows the rotor to operate at various speeds with an output frequency and voltage unrelated to rotor speed. The electrical system incorporates a unique inverter which yields the appropriate voltage and frequency in both the startup mode of operation as well as in the power generation mode of operation.
The electrical system is used to cause rotation of the turbine during the startup mode and subsequently is used to extract electrical power from the alternator after the turbine has reached its normal operating conditions. At startup, the alternator functions as an electric motor. The functions of the electrical system at startup comprise power boost, power switching and control to provide, for example, three-phase AC electrical power to the alternator. Both the frequency and voltage are controlled as a function of time and rotation speed. Electrical power for the electrical system is obtained during startup from either a DC source, such as a battery, or from an AC power line. The startup circuit may function as an open loop control system or as a closed loop control system based upon rotor position feedback.
As the turbine approaches normal operating conditions at very high speeds of rotation powered through the controlled combustion of fuel and air, the electronic circuitry used to initially drive the alternator as a motor is automatically reconfigured to accept power from the alternator. Subsequently, three-phase electrical power becomes available for extraction from the electrical system at desired voltages and frequencies.
Briefly, according to this invention, an electrical system for a turbine/alternator comprises a gas driven turbine and alternator rotating on a common shaft. The alternator has a permanent magnet rotor and a stator winding. A stator circuit is connected to the stator winding. A DC bus powers an inverter circuit. The output of the inverter circuit is connected to an AC output circuit or through a first contactor to the stator circuit. A rectifier is connected between the stator circuit and the DC bus. A signal generator is driven by signals derived from the rotation of the common shaft and an open loop waveform generator produces waveforms independent of the rotation of the common shaft. A second contactor connects either the signal generator or the open loop waveform generator to a driver connected to cause switching of the inverter circuit. A temporary power supply supplies energy to the DC bus. A control circuit, during a startup mode, switches the first contactor to connect the inverter circuit to the stator circuit and switches the second contactor to connect the signal generator to the driver, preferably a pulse width modulator. The control circuit, during a power out mode, switches the first contactor to disconnect the inverter from the stator circuit and switches the second contactor to connect the open loop waveform generator to the driver. During the startup mode, the alternator functions as a motor to raise the speed of the turbine to a safe ignition speed. The inverter is used to commutate the stator windings in response to the signal from the signal generator. During a power out mode, the inverter is used to convert the rectified output of the alternator into AC signals applied to the AC output circuit in response to the open loop waveform generator, thus producing electric power having a frequency unconnected to the rotational speed of the alternator.
According to a preferred embodiment, an electrical system for a turbine/alternator comprises a gas driven turbine and alternator rotating on a common shaft. The alternator is comprised of a permanent magnet rotor and a stator winding. The stator winding is connected through a contactor to an inverter circuit. The inverter circuit is connected to a DC bus. The inverter circuit is also connected to a signal generator. A position encoder is connected to the drive shaft of the turbine/alternator. Its output is also connected to the signal generator. The inverter processes the DC bus voltage and signal generator output to develop three-phase AC output voltages. The signal generator controls the inverter output frequency. Concurrently, a variable voltage DC power supply applies a time variant voltage to the DC bus. The DC bus voltage controls the inverter output voltage level. Thus, the output frequencies and voltages of the inverter are fully controllable. During the startup mode, the output of the inverter is applied through a contactor to the alternator which functions as an electric motor. When the startup mode is initiated, the DC power supply voltage begins to ramp up from 0 volts. The signal generator output frequency is set to a fixed low frequency. As the DC bus voltage begins to increase, the alternator rotor begins to rotate at a low speed. The encoder senses shaft position changes and sends this information to the signal generator. The signal generator processes this information and begins to ramp up its output frequency as a function of engine speed. This increasing frequency is directed to the inverter where it is used to control the frequency of the inverter output voltage. This controlled process results in a time variant inverter output whose frequency and voltage are applied through a contactor to the alternator. As a result, the alternator functions as a motor and accelerates the speed of the turbine shaft to a value suitable for ignition. Once the turbine has reached its normal operating speed, the variable voltage power supply is deactivated. Further, the shaft position encoder signal is disconnected from the signal generator and is replaced by a precision, fixed time base signal. Subsequently, the alternator AC output voltage is rectified and the resulting DC output voltages are applied to the DC bus. This reconfiguration permits the inverter to operate as a fixed frequency power output source independent of turbine rotor speed. In the power output mode, the inverter provides power through output filters. The filtered output power is then connected to a contactor which directs it to a set of terminals where it is available for consumer use. A control system integrates operation of the inverter, power supply, signal generator and contactors during both the startup and power output modes of operation. During the power output mode of operation, the control system continuously measures output voltages from the inverter and sends signals to the signal generator to compensate for output voltage fluctuations caused by varying output load conditions.
According to a preferred embodiment, the signal generator is a pulse width modulator. Typically, the stator winding of the alternator is a three-phase winding and the inverter circuit and the AC circuits are three-phase circuits.
According to a preferred embodiment, the electrical system comprises a battery powered supply circuit including a battery and a boost from 0 inverter circuit for outputting to the DC bus a voltage between 0 and that required by the inverter to power the alternator to safe ignition speeds. According to another preferred circuit, the battery powered supply circuit comprises a step-down circuit for recharging the battery and for powering low voltage devices such as fans and pumps from the DC bus during the output mode.